There presently exist various methods for the coating of paper with a plastic material. While coated paper of various forms has a wide variety of uses in the paper industry, in certain applications there is a need for a high quality of coated paper. For purposes of high quality printing such as for magazines, annual reports and the like, the quality of paper used in such printing can be assessed by the uniform and smooth nature of the coating on the paper. For example, in cases where the coating is absent in even very small microscopic amounts or dots, this will directly affect the quality of the product and in turn the accuracy of the printing thereon. Without a high quality finish on the paper, the end result of high quality printing thereon will not be achieved.
Various methods of coating paper have been described. U.S. Pat. No. 4,133,710, entitled "Method for Forming a Polyethylene Layer on a Substrate," issued Jan. 9, 1979, describes adhering extruded polyethylene film and a paper substrate. The film and paper in this method are pressed together by passing through rollers that cool the film. A liquid is used on the surface of the roller(s) to reduce the surface tension. The method is said to reduce adhesion to the roller while providing a more uniform surface structure of the plastic coated material.
U.S Pat. No. 3,911,191, entitled "Coated Paper Products," issued Oct. 7, 1975, discloses producing a high quality product by using a highly pigmented hot melt coating composition as compared to solvent or water based systems. While such disclosures refer generally to uniformity or quality of the resulting coated paper, they do not relate it to the effect it has on the subsequent printing thereon.
Accordingly, it is an object of the invention to provide a method of producing high quality plastic coated paper which is of sufficient smoothness and uniformity to allow for high quality printing thereon.
It is another object to provide a method of making such a quality product which includes reducing or eliminating gaps or defects (pits) in the coating which are of a size equal to or greater than the dot elements of conventional printing devices.
It is by this last object that the present method is distinctly effective in providing high quality polymer coated paper. The product of this method has a smoothness or evenness of the coating which enables high quality printing by allowing the wetting of ink in the shallower defects, and by reducing the number of missing print dots and unprinted areas. By reducing the defect or pit size and density in the coating, a much improved product is ultimately achieved. The present invention accomplishes this through a combination of steps. A highly smooth calendered paper sheet is subjected to extrusion coating with a polyolefin or other extrusion coating resin. The combination paper and coating are then subjected to a low-friction gloss chill roll finish. The surprising result of this combination is that the quality of the coated paper produced allows for a quality of printing thereon which is a substantial improvement compared to coated papers of comparable prior art processes.